When I set the MAC address of the interface with the command 'ifconfig wlan0 hw ether NN:NN:NN:NN:NN:NN' and NetworkManager is running, NetworkManager resets the MAC to its original value when later this interface is brought down at some point.

This doesn't happen without the NetworkManager. So if the NetworkManager service is stopped before and restarted after the MAC change, this problem doesn't occur.

But the fact that NetworkManager would reset the MAC is bothersome, and is indicative of the wrong behavior. Once the user changes the MAC, NetworkManager should not be resetting it since there is no reason for it to assert the original MAC. The most NetworkManager can do is to memorize the new MAC and to use it for whatever it is using them.

I searched for SIOCSIFHWADDR in NetworkManager sources and didn't find it. So most likely this is done from some library called from the NetworkManager.

Mint distro.